Microscope objective

ABSTRACT

A microscope objective comprising a first, second, third, fourth, fifth and sixth lens components wherein the first and second lens components are positive lenses respectively, the third lens component is a positive cemented lens, the fourth lens component is a cemented meniscus lens arranged to be convex toward the object side, the fifth lens component is a cemented meniscus lens arranged to be concave toward the object side, and the sixth lens component is a positive cemented doublet, the microscope objective having the magnification about 20X and N.A. about 0.4 and, at the same time, having a long working distance and flat image surface.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a microscope objective and, moreparticularly, to a microscope objective with the magnification about 20Xhaving a sufficiently large aperture as required, i.e., N.A. about 0.4,and still having a long working distance and flat image surface.

(b) Description of the Prior Art

As microscope objectives with flat image surface, it is known that thereare such lens systems in which a meniscus lens having a concave surfacewith a small radius of curvature on the object side is arranged as thefirst lens component, and lens systems in which a meniscus lens concavetoward the image side is arranged in a rear position. The objectivedisclosed in Japanese published unexamined patent application No.45741/72 which adoptes the Gauss type lens configuration is known as alens system having a still longer working distance. However, theobjective disclosed in said patent application has a five-componenteight-element lens configuration and, when the working distance is madelong in case of a lens system with a number of lens elements asmentioned in the above, rays cannot be converged unless the refractivepower of the first lens component is made strong. As a result, theradius of curvature of the first lens component becomes small andspherical aberration increases. When it is attempted to correctspherical aberration, the function for correction of aberrations shouldbe alloted to other lenses and, as a result, it becomes difficult tocorrect curvature of field, coma, etc. Due to the reason described inthe above, there is no known microscope objective of which the workingdistance is long and, at the same time, the image surface is flat over asufficiently wide field.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide amicroscope objective of which the working distance is remarkably long,image surface is flat over a sufficiently wide field and, at the sametime, aberrations are corrected satisfactorily favourably.

To compose a microscope objective with an ultra-long working distancewhich is an object of the present invention, it is necessary to shiftthe principal point toward the object side and, for that purpose, a lenshaving a strong negative power should be arranged on the image side.

The microscope objective according to the present invention is arrangedto have such lens configuration that a lens surface having a strongnegative power is arranged on the image side, at least seven surfaceshaving positive refractive powers are provided to positive lenscomponents arranged on the object side and, moreover, two surfacestherein are formed as approximately aplanatic surfaces, the microscopeobjective according to the present invention being thereby arranged toensure an ultra-long working distance by correcting aberrationsfavourably.

That is, the microscope objective according to the present inventioncomprises a first, second, third, fourth, fifth and sixth lenscomponents wherein the first lens component is a positive meniscus lensarranged to be concave toward the object side, the second lens componentis a positive meniscus lens, the third lens component is a positivecemented lens component consisting of two or three lens elements, thefourth lens component is a negative cemented meniscus lens arranged tobe convex toward the object side, the fifth lens component is a negativecemented meniscus lens having a concave surface with a strong refractivepower on the object side, and the sixth lens component is a positivecemented doublet, the microscope objective according to the presentinvention being arranged to fulfill the conditions (1) through (6) shownbelow:

(1) 2.5f<|r₁ /(n₁ -1)|<3.5f, r₁ <0

(2) 0.45f<r₁₀ <0.55f

(3) 0.45f<|r₁₁ |<0.65f, r₁₁ <0

(4) ν₅, ν₈ ≧65

(5) 10f<|f₅₆ |<50f

(6) n₉ -n₁₀ ≧0.18

where, reference symbol f represents the focal length of the lens systemas a whole, reference symbol f₅₆ represents the total focal length ofthe fifth and sixth lens compoennts, reference symbol r₁ represents theradius of curvature of the surface on the object side of the first lenscomponent, reference symbol r₁₀ represents the radius of curvature ofthe surface on the image side of the fourth lens component, referencesymbol r₁₁ represents the radius of curvature of the surface on theobject side of the fifth lens component, reference symbol n₁ representsthe refractive index of the first lens component, reference symbols n₉and n₁₀ respectively represent refractive indices of respective lenselements constituting the sixth lens component, reference symbol ν₅represents Abbe's number of the lens element on the object side in thefourth elns component, and reference symbol ν₈ represents Abbe's numberof the lens element on the image side in the fifth lens component.

The condition (1) is established in order to ensure the free workingdistance. If the radius of curvature r₁ of the surface on the objectside of the first lens component becomes smaller than the lower limit ofthe condition (1), the free working distance becomes small though it ispossible to ensure the optical working distance. If r₁ becomes largerthan the upper limit of the condition (1), said surface gets away fromthe aplanatic condition. As a result, spherical aberration becomesunfavourable and it is impossible to correct it by other lenses.

Both of the conditions (2) and (3) relate to correction of curvature offield. If r₁₀ and/or |r₁₁ | becomes larger than the upper limit of thecorresponding condition, Petzval's sum becomes large. As a result, theflatness of the image surface becomes unfavourable and it is impossibleto correct it by other lens surfaces. If r₁₀ and/or |r₁₁ | becomessmaller than the lower limit of the corresponding condition, comaincreases though Petzva's sum becomes small, and it is impossible tocorrect aberrations in a well balanced state.

The condition (4) is established in order to correct chromaticaberration. Unless chromatic aberration is supressed by using materialswith Abbe's numbers 65 or more for the convex lenses in the cementeddoublets constituting the fourth and fifth lens components, it isimpossible to correct chromatic aberration by other lenses. In otherwords, chromatic aberration becomes unfavourable if the condition (4) isnot fulfilled.

The condition (5) is established in order to correct coma. Coma becomesnegative when |f₅₆ | becomes larger than the upper limit of thecondition (5) and becomes positive when |f₅₆ | becomes smaller than thelower limit of the condition (5). In either case, it is impossible tocorrect coma by other lenses.

The condition (6) also serves to correct coma. If the value defined bythe condition (6) becomes smaller than the lower limit thereof, it isimpossible to correct coma which is caused by the lens componentsarranged in the front portion of the lens system and, consequently, comabecomes a large positive value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of Embodiment 1 of the microscopeobjective according to the present invention;

FIG. 2 shows a sectional view of Embodiment 2 of the microscopeobjective according to the present invention;

FIGS. 3 and 4 respectively show graphs illustrating aberration curves ofEmbodments 1 and 2 of the present invention; and

FIG. 5 shows a sectional view illustrating an example of an imaging lenssystem to be used together with a microscope objective which is designedto form an image in the position of infinite distance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of the microscope objective according to thepresent invention described so far are shown below.

    ______________________________________                                        Embodiment 1                                                                  ______________________________________                                        f = 1 , NA = 0.4 , WD = 1.334 , β = -20X                                 f.sub.56 = -12.99 , r.sub.1 /(n.sub.1 - 1) = -2.936                           r.sub.1 = -1.740                                                              d.sub.1 = 0.1668                                                                             n.sub.1 = 1.5927                                                                            ν.sub.1 = 35.29                               r.sub.2 = -1.134                                                              d.sub.2 = 0.0111                                                              r.sub.3 = -22.781                                                             d.sub.3 = 0.2446                                                                             n.sub.2 = 1.4970                                                                            ν.sub.2 = 81.61                               r.sub.4 = -1.639                                                              d.sub.4 = 0.0167                                                              r.sub.5 = 1.442                                                               d.sub.5 = 0.5449                                                                             n.sub.3 = 1.4970                                                                            ν.sub.4 = 81.61                               r.sub.6 = -1.277                                                              d.sub.6 = 0.2113                                                                             n.sub.4 = 1.6968                                                                            ν.sub.4 = 56.49                               r.sub.7 = -2.075                                                              d.sub.7 = 0.0111                                                              r.sub.8 =  0.790                                                              d.sub.8 = 0.4337                                                                             n.sub.5 = 1.4970                                                                            ν.sub.5 = 81.61                               r.sub.9 = -2.582                                                              d.sub.9 = 0.1001                                                                             n.sub.6 = 1.71736                                                                           ν.sub.6 = 29.51                               r.sub.10 = 0.500                                                              d.sub.10 = 0.4003                                                             r.sub.11 = -0.505                                                             d.sub.11 = 0.1001                                                                            n.sub.7 = 1.68893                                                                           ν.sub.7 = 31.08                               r.sub.12 = 19.856                                                             d.sub.12 = 0.3558                                                                            n.sub.8 = 1.497                                                                             ν.sub.8 = 81.61                               r.sub.13 = -0.650                                                             d.sub.13 = 0.1101                                                             r.sub.14 = 1.180                                                              d.sub.14 = 0.2780                                                                            n.sub.9 = 1.78472                                                                           ν.sub.9 = 25.71                               r.sub.15 = -24.911                                                            d.sub.15 = 0.4448                                                                            n.sub.10 = 1.53172                                                                          ν.sub.10 = 48.90                              r.sub.16 = 0.809                                                              ______________________________________                                        Embodiment 2                                                                  ______________________________________                                        f = 1 , NA = 0.4 , WD = 1.334 , β  = -20X                                f.sub.56 = 32.102 , r.sub.1 /(n.sub.1 - 1) = -3.469                           r.sub.1 = -2.5704                                                             d.sub.1 = 0.2124                                                                             n.sub.1 = 1.741                                                                             ν.sub.1 = 52.68                               r.sub.2 = -1.2798                                                             d.sub.2 = 0.0178                                                              r.sub.3 = 3.0020                                                              d.sub.3 = 0.3892                                                                             n.sub.2 = 1.497                                                                             ν.sub.2 = 81.61                               r.sub.4 = -2.5063                                                             d.sub.4 = 0.0111                                                              r.sub.5 = 2.0489                                                              d.sub.5 = 0.1334                                                                             n.sub.3 = 1.51821                                                                           ν.sub.3 = 65.04                               r.sub.6 = 0.8896                                                              d.sub.6 = 0.4378                                                                             n.sub.4 = 1.497                                                                             ν.sub.4 = 81.61                               r.sub.7 = -2.0670                                                             d.sub.6 = 0.1334                                                                             n.sub.4 = 1.7725                                                                            ν.sub.4 = 49.66                               r.sub.7 = -4.7178                                                             d.sub.7 = 0.0111                                                              r.sub.8 =  1.1023                                                             d.sub.8 = 0.3336                                                                             n.sub.5 = 1.497                                                                             ν.sub.5 = 81.61                               r.sub.9 = -4.5591                                                             d.sub.9 = 0.7499                                                                             n.sub.6 = 1.74                                                                              ν.sub.6 = 28.29                               r.sub.10 = 0.5034                                                             d.sub.10 = 0.2224                                                             r.sub.11 = -0.5871                                                            d.sub.11 = 0.1001                                                                            n.sub.7 = 1.72047                                                                           ν.sub.7 = 34.72                               r.sub.12 = 1.7822                                                             d.sub.12 = 0.3063                                                                            n.sub.8 = 1.497                                                                             ν.sub.8 = 81.61                               r.sub.13 = -0.8096                                                            d.sub.13 = 0.0111                                                             r.sub.14 = 2.5936                                                             d.sub.14 = 0.2891                                                                            n.sub.9 = 1.7495                                                                            ν.sub.9 = 35.27                               r.sub.15 = -0.8368                                                            d.sub.15 = 0.1334                                                                            n.sub.10 = 1.54771                                                                          ν.sub.10 = 62.83                              r.sub.16 = 4.6508                                                             ______________________________________                                    

In embodiments shown in the above, reference symbols r₁ through r₁₆respectively represent radii of curvature of respective lens surfaces,reference symbols d₁ through d₁₅ respectively represent thicknesses ofrespective lenses and airspaces between respective lenses, referencesymbols n₁ through n₁₀ respectively represent refractive indices ofrespective lenses, and reference sybmols ν₁ through ν₁₀ respectivelyrepresent Abbe's numbers of respective lenses.

Out of respective embodiments shown in the above, Embodiment 1 has thelens configuration as shown in FIG. 1, and the third lens componentthereof is arranged as a cemented doublet. Embodiment 2 has the lensconfiguration as shown in FIG. 2, and the third lens component thereofis arranged as a cemented lens component consisting of three lenselements. Both of said embodiments are microscope objectives which aredesigned to form an image in the position of infinite distance.Therefore, they are to be used in combination with an imaging lenssystem which has, for example, a lens configuration as shown in FIG. 5and numerical data as shown below.

    ______________________________________                                        r.sub.17 = 3.282                                                              d.sub.17 = 0.4447                                                                            n.sub.11 = 1.4875                                                                           ν.sub.11 = 70.15                              r.sub.18 = -12.536                                                            d.sub.18 = 0.2223                                                             r.sub.19 = -6.394                                                             d.sub.19 = 0.2223                                                                            n.sub.12 = 1.7400                                                                           ν.sub.12 = 28.29                              r.sub.20 = -13.591                                                            d.sub.20 = 1.6674                                                             r.sub.21 = 4.885                                                              d.sub.21 = 0.2001                                                                            n.sub.13 = 1.4875                                                                           ν.sub.13 = 70.15                              r.sub.22 = 2.184                                                              ______________________________________                                    

The numerical data of the imaging lens system shown in the aboverepresents the values when the focal length f of the lens system as awhole of the microscope objective according to Embodiment 1 is regardedas f=1.

As described so far and as shown by respective embodiments, themicroscope objective according to the present invention ensures anultra-long working distance by keeping N.A. equivalent to those of knownlong working distance microscope objectives and without causing decreasein the resolving power. The microscope objective according to Embodiment1 comprises ten lens elements and has a working distance of 12 mm andflat image surface over a wide field up to the field number of 27. Themicroscope objective according to Embodiment 2 is arranged to correctoffaxial chromatic aberration more favourably by using a cemented lenscomponent consisting of three lens elements.

Graphs of aberration curves of these embodiments, when combined with theimaging lens system shown in FIG. 5, are as shown in FIGS. 3 and 4.

I claim:
 1. A microscope objective comprising a first, second, third,fourth, fifth and sixth lens components wherein said first lenscomponent is a positive lens, said second lens component is a positivelens, said third lens component is a positive cemented lens, said fourthlens component is a cemented meniscus lens arranged to be convex towardthe object side, said fifth lens component is a cemented meniscus lensarranged to be concave toward the object side, and said sixth lenscomponent is a cemented meniscus lens, said microscope objective beingarranged to fulfill the following conditions:(1) 2.5f<|r₁ /(n₁-1)|<3.5f, r₁ <0 (2) 0.45f<r₁₀ <0.55f (3) 0.45f<r₁₁ <0.65f, r₁₁ <0 (4)ν₅, ν₈ ≧65 (5) 10f<|f₅₆ |<50f (6) n₉ -n₁₀ ≧0.18where, reference symbol frepresents the focal length of the lens system as a whole, referencesymbol f₅₆ represents the total focal length of the fifth and sixth lenscomponents, reference symbol r₁ represents the radius of curvature ofthe surface on the object side of the first lens component, referencesymbol r₁₀ represents the radius of curvature of the surface on theimage side of the fourth lens component, reference symbol r₁₁ representsthe radius of curvature of the surface on the object side of the fifthlens component, reference symbol n₁ represents the refractive index ofthe first lens component, reference symbols n₉ and n₁₀ respectivelyrepresent refractive indices of respective lens elements constitutingthe sixth lens component, reference symbol ν₅ represents Abbe's numberof the lens element on the object side in the fourth lens component, andreference symbol ν₈ represents Abbe's number of the lens element on theimage side in the fifth lens component.
 2. A microscope objectiveaccording to claim 1 wherein said third lens component is arranged as apositive cemented doublet.
 3. A microscope objective according to claim2, in which said microscope objective has the following numerical data:

    ______________________________________                                        f = 1 , NA = 0.4 , WD = 1.334 , β = -20X                                 f.sub.56 = -12.99 , r.sub.1 /(n.sub.1 - 1) = -2.936                           r.sub.1 = -1.740                                                              d.sub.1 = 0.1668                                                                             n.sub.1 = 1.5927                                                                            ν.sub.1 = 35.29                               r.sub.2 = -1.134                                                              d.sub.2 = 0.0111                                                              r.sub.3 = -22.781                                                             d.sub.3 = 0.2446                                                                             n.sub.2 = 1.4970                                                                            ν.sub.2 = 81.61                               r.sub.4 = -1.639                                                              d.sub.4 = 0.0167                                                              r.sub.5 = 1.442                                                               d.sub.5 = 0.5449                                                                             n.sub.3 = 1.4970                                                                            ν.sub.3 = 81.61                               r.sub.6 = -1.277                                                              d.sub.6 = 0.2113                                                                             n.sub.4 = 1.6968                                                                            ν.sub.4 = 56.49                               r.sub.7 = -2.075                                                              d.sub.7 = 0.0111                                                              r.sub.8 = 0.790                                                               d.sub.8  = 0.4337                                                                            n.sub.5 = 1.4970                                                                            ν.sub.5 = 81.61                               r.sub.9 = -2.582                                                              d.sub.9 = 0.1001                                                                             n.sub.6 = 1.71736                                                                           ν.sub.6 = 29.51                               r.sub.10 = 0.500                                                              d.sub.10 = 0.4003                                                             r.sub.11 = -0.505                                                             d.sub.11 = 0.1001                                                                            n.sub.7 = 1.68893                                                                           ν.sub.7 = 31.08                               r.sub.12 = 19.856                                                             d.sub.12 = 0.3558                                                                            n.sub.8 = 1.497                                                                             ν.sub.8 = 81.61                               r.sub.13 = -0.650                                                             d.sub.13 = 0.1101                                                             r.sub.14 = 1.180                                                              d.sub.14 = 0.2780                                                                            n.sub.9 = 1.78472                                                                           ν.sub.9 = 25.71                               r.sub.15 = -24.911                                                            d.sub.15 = 0.4448                                                                            n.sub.10 = 1.53172                                                                          ν.sub.10 = 48.90                              r.sub.16 = 0.809                                                              ______________________________________                                    

where, reference symbols r₁ through r₁₆ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₁₅respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁ through n₁₀ respectivelyrepresent refractive indices of respective lenses, and reference symbolsν₁ through ν₁₀ respectively represent Abbe's numbers of respectivelenses.
 4. A microscope objective sccording to claim 1 wherein saidthird lens component is arranged as a positive cemented lens consistingof three lens elements.
 5. A microscope objective according to claim 4,in which said microscope objective has the following numerical data:

    ______________________________________                                        f = 1 , NA = 0.4 , WD = 1.334 , β = -20X                                 f.sub.56 = 32.102 , r.sub.1 /(n.sub.1 - 1) = -3.469                           r.sub.1 = -2.5704                                                             d.sub.1 = 0.2124                                                                             n.sub.1 = 1.741                                                                             ν.sub.1 = 52.68                               r.sub.2 = -1.2798                                                             d.sub.2 = 0.0178                                                              r.sub.3 = 3.0020                                                              d.sub.3 = 0.3892                                                                             n.sub.2 = 1.497                                                                             ν.sub.2 = 81.61                               r.sub.4 = -2.5063                                                             d.sub.4 = 0.0111                                                              r.sub.5 = 2.0489                                                              d.sub.5 = 0.1334                                                                             n.sub.3 = 1.51821                                                                           ν.sub.3 = 65.04                               r.sub.6 = 0.8896                                                              d.sub.6 = 0.4378                                                                             n.sub.4 = 1.497                                                                             ν.sub.4 = 81.61                               r.sub.7 = -2.0670                                                             d.sub.6 = 0.1334                                                                             n.sub.4 = 1.7725                                                                            ν.sub.4 = 49.66                               r.sub.7 =  -4.7178                                                            d.sub.7 = 0.0111                                                              r.sub.8 = 1.1023                                                              d.sub.8 = 0.3336                                                                             n.sub.5 = 1.497                                                                             ν.sub.5 = 81.61                               r.sub.9 = -4.5591                                                             d.sub.9 = 0.7499                                                                             n.sub.6 = 1.74                                                                              ν.sub.6 = 28.29                               r.sub.10 = 0.5034                                                             d.sub.10 = 0.2224                                                             r.sub.11 = -0.5871                                                            d.sub.11 = 0.1001                                                                            n.sub.7 = 1.72047                                                                           ν.sub.7 = 34.72                               r.sub.12 = 1.7822                                                             d.sub.12 = 0.3063                                                                            n.sub.8 =1.497                                                                              ν.sub.8 = 81.61                               r.sub.13 = -0.8096                                                            d.sub.13 = 0.0111                                                             r.sub.14 = 2.5936                                                             d.sub.14 = 0.2891                                                                            n.sub.9 = 1.7495                                                                            ν.sub.9 = 35.27                               r.sub.15 = -0.8368                                                            d.sub.15 = 0.1334                                                                            n.sub.10 = 1.54771                                                                          ν.sub.10 = 62.83                              r.sub.16 = 4.6508                                                             ______________________________________                                    

where, reference symbols r₁ through r₁₆ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₁₅respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁ through n₁₀ respectivelyrepresent refractive indices of respective lenses, and reference symbolsν₁ through ν₁₀ respectively represent Abbe's numbers of respectivelenses.